The embodiments relate generally to methods and apparatus for movement of equipment in passages. More particularly, the embodiments relate to a propulsion system for pulling casing into boreholes.
The art of drilling vertical, inclined, and horizontal boreholes plays an important role in the oil and gas industry. For example, a typical oil or gas well comprises a vertical borehole that is drilled by a rotary drill bit attached to the end of a drill string. The drill string is typically constructed of a series of connected links of drill pipe that extend between surface equipment and the drill bit. A drilling fluid, such as drilling mud, is pumped from the surface through the interior surface or flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the drill bit, and remove debris and rock chips from the borehole created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the space between the outer surface of the drill pipe and the inner surface of the borehole.
Conventional drilling often requires drilling numerous boreholes to recover hydrocarbons, such as gas and oil, or mineral deposits. For example, drilling for oil and gas usually includes drilling a vertical borehole until the reservoir is reached. The hydrocarbons are then pumped from the reservoir to the surface. As known in the industry, often a large number of vertical boreholes must be drilled within a small area to recover the hydrocarbons within the reservoir. This requires a large investment of resources and equipment and is very expensive. Additionally, the hydrocarbons within the reservoir may be difficult to recover for several reasons. For instance, the size and shape of the formation, the depth at which the hydrocarbons are located, and the location of the reservoir may make exploitation of the reservoir very difficult. Further, drilling for oil and gas located under bodies of water, such as the North Sea, often presents greater difficulties.
In order to recover hydrocarbons from these difficult to exploit reservoirs, it may be desirable to drill a borehole that is not vertically orientated. For example, the borehole may be initially drilled vertically downwardly to a predetermined depth and then drilled at an inclination to vertical to the desired target location. In other situations, it may be desirable to drill an inclined or horizontal borehole beginning at a selected depth. This allows the hydrocarbons located in difficult-to-reach locations to be recovered.
While several methods of drilling are known in the art, two frequently used methods to drill vertical, inclined, and horizontal boreholes are generally known as rotary drilling and coiled tubing drilling. In rotary drilling, a drill string, consisting of a series of connected segments of drill pipe, is lowered from the surface using surface equipment such as a derrick and draw works. Attached to the lower end of the drill string is a bottom hole assembly (“BHA”). The BHA typically includes a drill bit and may include other equipment known in the art such as drill collars, stabilizers, and heavy-weight pipe. The other end of the drill string is connected to a rotary table or top drive system located at the surface. The top drive system rotates the drill string, the BHA, and the drill bit, allowing the rotating drill bit to penetrate into the formation. The direction of the rotary drilled borehole can be gradually altered by using known equipment such as a downhole motor with an adjustable bent housing to create inclined and horizontal boreholes.
Another type of known drilling is coiled tubing drilling. In coiled tubing drilling, the drill string tubing is fed into the borehole by an injector assembly. In contrast to rotary drilling, the drill string is not rotated. Instead, a downhole motor as part of the BHA provides rotation to the drill bit. Because the coiled tubing is not rotated or used to force the drill bit into the formation, the strength and stiffness of the coiled tubing is typically much less than that of the drill pipe used in comparable rotary drilling. Thus, the thickness of the coiled tubing is generally less than the drill pipe thickness used in rotary drilling, and the coiled tubing generally cannot withstand the same rotational and tension forces in comparison to the drill pipe used in rotary drilling.
The use of coiled tubing drilling typically eliminates the use of conventional rigs and conventional drilling equipment. See for example U.S. Pat. Nos. 5,215,151; 5,394,951 and 5,713,422, all hereby incorporated herein by reference. The BHA may also include a propulsion system that propels the bit down the borehole. One such propulsion system is a thruster that pushes off the lower terminal end of the coiled tubing and does not rely upon contacting or gripping the inside wall of the borehole.
Another such self-propelled propulsion system is manufactured by Western Well Tool. The propulsion system includes an upper and lower housing with a packerfoot mounted on each end. Each housing has a hydraulic cylinder and ram for moving the propulsion system within the borehole. The propulsion system operates by the lower packerfoot expanding into engagement with the wall of the borehole with the ram in the lower housing extending in the cylinder to force the bit downhole. Simultaneously, the upper packerfoot contracts and moves to the other end of the upper housing. Once the ram in the lower housing completes its stroke, then the hydraulic ram in the upper housing is actuated to propel the bit and motor further downhole as the lower packerfoot contracts and resets at the other end of the lower housing. This cycle is repeated to continuously move the BHA within the borehole. The propulsion system can propel the BHA in either direction in the borehole. Flow passages are provided between the packer-feet and housings to allow the passage of drilling fluids through the annulus formed by the coiled tubing and borehole.
Various companies manufacture other types of self-propelled propulsion systems for propelling the bit and pulling steel coiled tubing in the well. These propulsion systems include self-propelled wheels that frictionally engage the wall of the borehole. However, there is very little clearance between the wheels of the propulsion system and the wall of the borehole and problems arise when the wheels encounter ridges or other variances in the dimensions of the wall of the borehole. Further, at times there is an inadequate frictional engagement between the wheels and the wall of the borehole to adequately propel the propulsion system.
Other companies also offer propulsion systems to “walk” the end of a wireline down a cased borehole. However, these propulsion systems engage the interior wall of a casing having a known inside dimension. One such propulsion system is manufactured by Schlumberger.
Another form of drilling is composite tubing drilling. Similar to coiled tubing drilling, a propulsion system can also be used with composite tubing to drill a borehole. An example of a drilling system using a propulsion system with composite coiled tubing is U.S. Pat. No. 6,296,066, hereby incorporated herein by reference. With composite tubing drilling, instead of using coiled metal tubing, composite coiled tubing is used as the drilling conduit for transfer of the drilling fluids. With composite tubing, the drill string is also not rotated.
For all of the methods of drilling discussed above, during the course of the drilling program, the borehole typically has one or more “casing strings” run and cemented in place. A typical drilling program first involves drilling a large diameter borehole from the earth's surface for several thousand feet. A “surface casing” string is then run into the borehole and cemented in place. After the cement in the annulus has cured or hardened, another drill bit is utilized to drill through the cement in the surface casing to drill a second and deeper borehole into the earth formations. Typically, the subsequent drill bit has a smaller diameter that the initial drill bit such that the second borehole has a smaller diameter than the diameter of the surface borehole. However, it should be appreciated that bi-center bits and wing reamers may be used to enlarge the diameter of the second borehole.
With respect to the section of borehole subsequently drilled below a surface casing, at an appropriate depth, the drilling of the borehole is discontinued and a string of pipe commonly called a casing or liner is inserted through the surface casing. As a matter of nomenclature, a liner is a string of pipe typically suspended in the lower end of the previously set casing by a liner hanger so that the lower end of the liner does not touch the bottom of the borehole and the liner thus is suspended under the tension of the pipe weight on the liner hanger. In some instances, a liner is set on the bottom of the borehole but its upper end does not extend to the earth's surface.
If the pipe set in the borehole subsequently drilled extends to the surface of the earth it is also called a casing. When the cementing operation is completed and the cement sets, there is a column of cement in the annulus of the subsequent string of pipe. The casing strings are usually comprised of a number of joints, each being on the order of forty feet long, connected to one another by threaded connections or other connection means. Also, the joints are typical metal pipes, but may also be non-metal materials such as composite tubing.
Typically, the casing string is merely gravity fed into a vertical borehole. If a top drive rig is used, the rig can hydraulically force the casing string down into the borehole. If gravity fed, however, the weight of the casing is used to install the casing in the borehole. Typically, a casing shoe is disposed on the lower end of the casing string to close off the lower end of the casing string. The casing shoe closes off the lower end of the string so that the casing then serves as a pressure vessel in which fluid pressure can be applied to help force the casing down hole. The shoe typically is bullet shaped with a spherical-type face. A float valve may be attached to the lower end of the casing that allows the fluid to pass down the casing and out through the lower end to allow fluid circulation.
The advent in recent years of highly deviated or horizontal wells in the oil and gas industry has increased both the frequency and seriousness of difficulties encountered while running borehole casing strings. Particularly, problems occur in a borehole that has an extended reach horizontal portion. Horizontal wells may be at shallow depths where the vertical portion of the well is small. With a small vertical portion, the vertical length of the casing is short whereby minimum weight is provided by the drill string to allow gravity to assist in setting the casing. In addition, in a horizontal well, the drag becomes so great on the casing string that it can no longer be forced into the borehole. Also, if a borehole has high build rates, such as 30° per hundred feet plus, there can be a wash out in the curved section. If there is a wash out, the end of the pipe may tend to bury itself into the wash out portion rather than follow the bends or curves in the borehole. Thus, the end of the pipe could dead end into one of the cavities caused by the wash out rather than make the turn in the borehole.
Another prior art solution to these problems includes floating the pipe by making the string of casing a closed vessel and either filling the casing with a low density fluid or possibly only having air in the casing. The borehole is filled with fluid to place a column on the well to maintain control. The fluid inside the casing has a lower density than the fluid forming the column in the annulus and causes the casing string to tend to be buoyant and “float” in the borehole fluid. Causing the casing string to float reduces the drag on the highly deviated borehole wall. This methodology, however, is delicate because of the collapse pressure of the casing. The casing will collapse if the pressure differential across the casing wall becomes too great. In any event, floating the casing still does not completely eliminate the drag on the casing and thus the methodology is still subject to the problems discussed above for non-floating casing.
The consequence of encountering such difficulties are, at best, delays in the schedule of the well program and, at worst, having to drill all or part of the well again. In any case, significant additional cost is involved. Thus, there exists a need for an apparatus and method of installing casing into highly-deviated and horizontal boreholes. The casing must thus be able to maneuver through curves in the borehole. The casing must also be able to be installed in boreholes of great length, in the order of 50,000 feet. The apparatus and method of installing the casing must also cost-effectively install casing into the borehole. The cost-effectiveness not only takes into consideration the resources needed to install the casing, but also the amount of time required.
Other objects and advantages of the invention will appear from the following description.